1. Field of the Invention
This invention relates to catalyst carriers to be used as supports for metal and metal oxide catalyst components which are then used in a variety of chemical reactions. More specifically, the invention pertains to a process of formulating catalyst having a low surface area alpha alumina carrier that is suitable as a support for silver, and the use of such catalyst in chemical reactions, especially the epoxidation of ethylene to ethylene oxide.
2. Description of the Related Art
Alumina is well known to be useful as a catalyst support for the epoxidation of olefins. It is particularly useful as a support for a catalyst comprising silver which is employed in the oxidation of ethylene to ethylene oxide. Support materials are made by fusing high purity aluminum oxide, with or without silica. For this purpose the support material often comprises 90 percent or more, by weight, alpha alumina and up to 6 percent, by weight, silica. They are usually very porous and have a high or low surface area depending on the use to be made of them.
In known processes of making a support, alpha alumina and/or transition alumina (alpha alumina precursors) is thoroughly mixed with temporary and permanent binders. The temporary binders hold together the components of the carrier precursor during its processing. The permanent binders are inorganic materials having fusion temperatures below that of the alumina and induce fusion at the points of contact of the alumina particles, and impart mechanical strength to the finished support. After thorough dry-mixing, sufficient water is added to the mass to form the mass into a paste-like extrudable admixture. The catalyst support particles are then formed from the paste by conventional means such as high pressure extrusion, tableting, granulation or other ceramic forming processes. The particles are then dried and are subsequently fired at an elevated temperature.
In the firing step, the temporary binders are burnt or thermally decomposed to carbon dioxide and water, and are volatilized. It is known in the art that ceramic carrier based catalysts comprise inert, solid supports such as alpha alumina. Such have been described in U.S. Pat. Nos. 3,664,970; 3,804,781; 4,428,863 and 4,874,739. U.S. patents which describe the making of alumina supports include U.S. Pat. Nos. 2,499,675; 2,950,169 and 3,172,866. Such carriers have potential applications in the catalytic field especially where the alumina base is alpha alumina. Other patents such as U.S. Pat. Nos. 3,222,129; 3,223,483 and 3,226,191 show the preparation of active aluminas. Methods of making highly porous aluminas are disclosed in U.S. Pat. Nos. 3,804,781; 3,856,708; 3,907,512 and 3,907,982. Alumina carriers having high thermal stability are disclosed in U.S. Pat. No. 3,928,236. Other methods of making catalyst carriers are discussed in U.S. Pat. Nos. 3,987,155; 3,997,476; 4,001,144; 4,022,715; 4,039,481; 4,098,874 and 4,242,233. U.S. Pat. No. 3,664,970 discloses a carrier containing mainly alumina and also contains silica, magnesia and titania. U.S. Pat. No. 4,410,453 discloses that the performance of a silver on alumina catalyst for the oxidation of ethylene to ethylene oxide is improved by the inclusion of an oxide, or oxide precursor, of zinc, lanthanum, or magnesium. U.S. Pat. No. 4,200,552 discloses a carrier that is made of α-alumina and at least one of the compounds SiO2, TiO2, ZrO2, CaO, MgO, B2O3, MnO2, or Cr2O3, as a sintering agent. U.S. Pat. No. 4,455,392 discloses the composition of an alumina carrier that contains silica and magnesia as components of the bonding material. U.S. Pat. No. 5,100,859 discloses a carrier that contains an alkaline earth metal silicate, which may be added as an original component or generated in situ by the reaction of silica, or silica generating compounds, with compounds that decompose to alkaline earth metal oxide upon heating. U.S. Pat. No. 5,512,530 discloses a process for the production of a catalyst carrier which is based on mixing alpha alumina, burnout material, and titania. U.S. Pat. No. 5,380,697 discloses a carrier containing a ceramic bond comprises 60% wt. silica, 29% wt. alumina, 3% wt. calcium oxide, 2% magnesia, 4% wt. alkali metal oxides and less than 1% wt. each of ferric oxide and titania. U.S. Pat. No. 5,733,840 and U.S. Pat. No. 5,929,259 disclose a titania-modification of formed carriers. The treatment involved impregnating the pre-formed carrier with a solution of titanyl oxalate, titanium (IV) bis(ammonium lactato)dihydroxide, or similar organic salts and then the impregnated carrier is calcined at a temperature from about 450 to 700° C. The patents disclosed that if titania is added during the carrier's preparation, it tend to affect the densification of the carrier structure which can lead to unacceptable properties. U.S. Pat. No. 4,368,144 states that better catalytic performance was obtained with carriers that contain no more than 0.07% Na. U.S. Pat. No. 6,103,916 patent discloses that catalyst performance was improved when the carrier was washed by boiling in pure water until the water resistivity is more than 10,000 Ω·cm. U.S. Pat. No. 5,384,302 patent disclosed an α-alumina based carrier which prepared by mixing at least two alumina components. The first provides 95-40% of the total aluminum components and is made of α-alumina of crystallite size of 0.4-4μ.
One of the problems with the catalysts that are based on porous carriers is that they have an insufficiently uniform pore structure. U.S. Pat. No. 4,022,715 attempts to solve this problem by using an organic solution of a blowing agent, mixed with a carrier precursor composition. It has now been found that an improved carrier pore structure can be formed by employing a composition for a catalyst support which comprises an admixture of a transition alumina alone with or without an alpha alumina having a median particle size of about 5μ or greater; and a binder. The composition is substantially free of seeding particles. Optionally the composition may have either a solid blowing agent which expands, or propels a gas upon the application of sufficient heat, talc and/or a water soluble titanium compound.
The catalyst support of this invention has excellent crush strength, porosity, and surface area on which the catalytic component will be deposited. The optimum porosity and surface area are important for catalytic function and insures the absence of diffusional resistances for reactants and product gases under reaction conditions. A minimum surface area is important because it provides the structure on which the catalytic component will be deposited. Crush strength is a parameter of the physical integrity of the carrier. This physical strength is essential for the catalyst ability to withstand handling as well as its long life in a commercial reactor. A carrier that has the optimum surface area and porosity may be deficient in its crush strength, and vice versa. The balance between the different physical specifications of the carrier is important. Normally, a carrier that has the optimum surface area and porosity may be deficient in its crush strength, and visa-versa. The proper choice of the specifications of the alpha alumina, that is used in the carrier composite, will help in the balance between the properties of the finished carrier. An optimum balance between these specifications is obtained via the choice of the carrier components according to the invention.